In the case of abrading silicon wafers, the silicon wafers are abraded by, for example, an abrasive machine 10 shown in FIG. 2. In the abrasive machine 10, abrasive cloth 14 is adhered on a rotating abrasive plate 12. A silicon wafer 16 is pressed onto the abrasive cloth 14 by an abrasive head 20 so that a surface of the silicon wafer 16 can be abraded. Slurry including abrasive grains is supplied to the surface of the silicon wafer 16, and the used slurry is collected to be reused.
Namely, the slurry, in which abrasive grains are mixed, is dropped onto the abrasive cloth 14 so as to abrade the surface of the wafer 16, then the slurry is discharged from the abrasive cloth 14 to a collecting section 18 which is provided outside of the abrasive plate 12. The slurry discharged to the collecting section 18 has been heated by friction between the surface of the wafer 16 and the abrasive cloth 14, so the discharged slurry must be cooled, by a heat exchanger “H”, until reaching a prescribed temperature. Then, abraded dusts included in the discharged slurry, which has been cooled, are removed by a removing unit 22. The slurry, from which the abraded dusts have been removed, is stored in a tank 24, and the slurry in the tank 24 is supplied to the abrasive cloth 14 again, by a pump 26, via an electromagnetic valve 28.
By providing the heat exchanger “H” in a circulation circuit of the slurry, the temperature of the slurry in the tank 24 can be maintained at a prescribed temperature, and the silicon wafers 16 can be abraded at a fixed abrasive rate without heat-deformation of the abrasive plate 12. In some cases, etching liquid is used as the machining liquid. Generally, the etching function of the etching liquid highly depends on temperature. If the temperature of the etching liquid is high, the etching function is sharply increased, so it is difficult to control the etching rate.
The abrasive plate 12 is heated by frictional heat between the surface of the wafer 16 and the abrasive cloth 14, and the abrasive plate 12 deforms when the abrasive plate 12 is overheated, so that accuracy of abrading the surface of the wafer 16 becomes low.
By providing the heat exchanger “H” so as to maintain the temperature of the slurry in the tank 24, the sharp increase of the etching function can be prevented, so that the etching rate can be easily controlled. Further, the heat of the liquid supplied to the abrasive plate 12 can be removed, so that the heat-deformation of the abrasive plate 12 can be prevented. The wafers 16 can be stably abraded with high abrasive accuracy.
A conventional heat exchanger “H” is shown in FIG. 5. The heat exchanger 180 is a double-tube type including: an inner heat exchanging tube 100 in which the discharged slurry flows; and an outer tube 102 in which cooling water flows along an outer circumferential face of the inner heat exchanging tube 100. The inner heat exchanging tube 100 is a fluororesin tube or a stainless tube coated with fluororesin and the outer tube 102 is made of vinyl chloride. As clearly shown in FIG. 5, an inlet 104 and an outlet 106 of the discharged slurry, which are provided to the heat exchanging tube 100, and an inlet 108 and an outlet 110 of the cooling water, which are provided to the outer tube 102, are arranged so as to flow the discharged slurry and the cooling water as countercurrents.
In the abrasive machine shown in FIG. 3, which has the heat exchanger “H”, the discharged slurry heated by the frictional heat can be cooled. Even if the slurry is circulated to reuse, the wafers 16 can be stably abraded.
However, heat conductivity of the heat exchanging tube 100 made of a fluororesin is low. Therefore, a broad heat conductive area is required so as to properly remove the heat, with the result that the heat exchanger 180 must be large. If the heat exchanger 180 is large, the residence time of the machining liquid in the heat exchanger 180 must long, so that accuracy of controlling the temperature of the machining liquid, e.g., slurry, etching liquid, is low, the abrasive plate 12 deforms, and the etching function of the etching liquid is adversely affected.
In the case of the stainless heat exchanging tube which is not coated with fluororesin, the heat conductivity is high, so the heat conductive area can be small and size of the heat exchanger can be small.
However, metal ions solved out from the stainless tube stick onto the surface of the silicon wafer 16 to be abraded so that the function of the semiconductor chips is adversely affected.